Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Fibre reinforced polymer composites are known in the art and are commonly made by reacting a curable resin with a reactive diluent in the presence of a free radical initiator. Typically, the curable resin is an unsaturated polyester resin and the reactive diluent is a vinyl monomer. Reinforcing materials such as glass fibre are often included in the formulations to provide dimensional stability and toughness. Such reinforced composites are used in many key industrial applications, including: construction, automotive, aerospace, marine and for corrosion resistant products.
For traditional glass fibre reinforced polymer composites, the fibre lengths typically range from about 12 mm up to tens of meters in the case of, for example, filament winding. In these glass fibre polymer composites the majority of fibres are held in position by mechanical friction and there is only relatively weak bonding of the fibres to the resin matrix. Therefore, the performance of such polymer composites is largely due to the length of the fibres employed and in these composites there is a discontinuity/gap between the fibres and the resin. Cracks initiated in the resin matrix find it very difficult to jump gaps, therefore in these composites cracks initiated in the resin are usually arrested at the resin boundary and do not reach the glass surface. However, traditional glass fibre composites have a number of shortcomings. For example, it is difficult to “wet” the fibres with the resin prior to curing, and even dispersion of long fibres throughout the composite is difficult, especially for complex parts.
In addition, such traditional glass reinforced polymer composites are limited by their production techniques which generally require manual layering or are extremely limited in the shape and complexity of the moulds.
To overcome these shortcomings, very short glass fibres may be used. VSFPLCs or very short fibre polymerisable liquid composites can product laminate with tensile strengths greater than 80 MPa flexural strength greater than 130 MPa. VSFPLCs are suspension of very short surface treated reinforcing fibres and polymerisable resins/thermoset such as UP resins vinyl functional resins, epoxy resins or polyurethane resins. The length of the fibres are kept very short so that they do not increase the viscosity of the liquid to where the resin fibre mixture is no longer sprayable or pumpable. VSFPLCs can be used to replace standard fibre glass layouts in open and closed moulding applications and also can be used as alternatives to thermoplastics in resin injection moulding and rotation moulding applications.
However, an improvement in the fibre-to-matrix bond is typically required since such very short glass fibres are too short to be mechanically “keyed” into the matrix. Coating the reinforcing fibre with a coupling agent may provide an improvement in the fibre-to-matrix bond. For example, one commonly used coupling agent is Dow Corning Z-6030, which is a bifunctional silane containing a methacrylate reactive organic group and 3 methoxysilyl groups. Dow Corning Z-6030 reacts with organic thermoset resins as well as inorganic minerals such as the glass fibre. Whilst such coupling agents may improve the fibre-to-matrix bond, the usefulness of the reinforced polymer composite is limited since they are prone to embrittlement over time. A product with greater flexibility and toughness is sometimes needed.
An attempt was made to address some of these shortcomings in PCT Patent Application No. PCT/AU01/01484 (International Publication No. WO 02/40577) where the coupling agent was pre-polymerised prior to coating the glass reinforcing fibre to “plasticise the interface”. The intention of the pre-polymerised coupling agent was to provide a rubbery interphase between the fibre and the bulk resin and thereby result in product having improved impact resistance and strength. However, long-term embrittlement is still an issue with the above PCT. In Very Short Fibre Polymerisable Liquid Composites there are no air gaps between the fibre and the resin. In VSFPLCs the resin is chemically bonded to the resin matrix and there are no gaps between the resin and the fibres. Cracks initiated in the resin matrix travel directly to the fibre surface. All the energy of the propagating crack is focused at a point on the glass fibre, and the energy is sufficient to rupture the fibre. Abundant evidence for this can be seen on the fracture surface of silane treated fibres. This is especially true for laminates with flexural strengths greater than 100 MPa.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the abovementioned prior art, or to provide a useful alternative.